empymod/empymod: empymod v1.7.3

Small improvements related to speed as a result of the benchmarks introduced in v1.7.2:

• Kernels which do not exist for a given ab are now returned as None from kernel.wavenumber instead of arrays of zeroes. This permits for some time saving in the transforms. This change is backwards incompatible if you directly used kernel.wavenumber. Nothing changes for the user-facing routines in model.

• Adjustments in transform with regard to the None returned by kernel.wavenumber. The kernels are not checked anymore if they are all zeroes (which can be slow for big arrays). If they are not None, they will be processed.

• Various small improvements for speed to transform.dlf (i.e. factAng; log10/log; re-arranging).